1. Field of the Invention
This invention relates to a motor drive control technique, and more particularly, to a circuit for controlling the drive current of the motor such that it does not exceed a predetermined target value.
2. The Prior Art
FIG. 1 shows a conventional drive control device to which this invention relates.
An input terminal 2 for receiving a current instruction signal, V.sub.i, is connected through a current amplifier 3 to first input terminals of two PWM (pulse width modulation) type comparators 4 and 5, the other input terminals of which are connected to an input terminal 6 for receiving a triangular wave signal V.sub.t. The output terminals of the comparators 4 and 5 are connected to the control input terminals of switching elements 7, 8, 9 and 10 such as FETs. These switching elements 7, 8, 9 and 10 form a bridge circuit 13 functioning as a reversible chopper for selectively suppling current from a power source terminal 11 through a DC motor 14 in either the forward or reverse direction to ground 2.
The DC motor 14 to be controlled is connected between the mid-points of the bridge circuit 13, that is, between the node connecting switching elements 7 and 10, and the node connecting switching elements 8 and 9. The bridge circuit 13 includes current detectors 15 and 16, such as current transformers, connected, respectively, between ground and switching element 8, and ground and switching element 10. The current detectors 15 and 16 are also connected to a current detecting amplifier 17, the output of which is connected to an absolute value comparator 18. The output terminal of the absolute value comparator 18 is connected directly to a switch circuit 20 and, through an inverter circuit 19, to a switch circuit 21. First terminals of the switch circuits 20 and 21 are connected to the input terminal of the current amplififer 3, and the other terminals are connected, respectively, through time-constant adjusting devices 22 and 23 to the output terminal of the current amplifier 3.
The current instruction signal V.sub.i is applied to the current amplifier 3. The output of the current amplifier 3 is applied directly to a first input terminal of the comparator 4, and is also applied to a first input terminal of the comparator 5 through an inverter. That is, each of the comparators 4 and 5 receives the current instruction signal V.sub.i through its first input terminal and the triangular wave signal V.sub.t through its second input terminal. The comparators subject these signals to a comparison to provide a drive signal V.sub.O having a pulse width proportional to the current instruction signal V.sub.i. The output drive signal V.sub.O of the comparator 4 is applied to the control input terminals of the switching elements 7 and 8, and the output drive signal V.sub.O of the comparator 5 is applied to the control input terminals of the switching elements 9 and 10. Therefore, either the switching elements 7 and 8, or 9 and 10 of the bridge circuit 13 are simultaneously conducting to drive the DC motor 14 at a predetermined speed by controlling the value and direction of the drive current I passing through the DC motor 14.
The drive current I is detected by the current detectors 15 and 16, amplified by the amplifier 17, and compared with a reference value S. When the drive current I becomes excessively large as determined by the reference value S, the absolute value comparator 18 turns on the switch circuit 21 while turning switch circuit 20 off to thereby change the time constant and gain of current amplifier 3. As shown in FIG. 2, when the switch circuit 20 is turned on, the drive circuit transfer function is A.multidot.Gm/(1+A.multidot.Gm.multidot.Gfb), and when the switch circuit 21 is turned on, the transfer function is X.multidot.A.multidot.Gm/(1+X.multidot.A.multidot.Gm.multidot.Gfb).
In this way, the gain and the time constant of the current amplifier 3 are multiplied by a factor X (where X is larger than 0 and smaller than 1). As a result, the current amplifier 3 operates to decrease the drive current I. Thus, owing to the gain and time constant of the current amplifier 3 and the hysteresis characteristic of the absolute value comparator 13, the drive control device of FIG. 1 causes the drive current I to repeatedly oscillate about the target value in a step-wise fashion.
In the conventional drive control device, the switch circuits 20 and 21 and the time constant adjusting devices 22 and 23 are externally connected to the current amplifier 3. Therefore, the conventional drive control device is intricate, and difficult to adjust to an ideal setting. Furthermore, as it is essential that the circuit time constants are large, the device response speed is low. In addition, the peak current is high, which makes it difficult to control the current with high accuracy. Still further, the device, being affected by noise, may become unstable.